Chargeable/dischargeable secondary batteries are broadly divided into a hard pack type and inner pack type. A typical example of the hard pack battery is shown in FIG. 1. Referring to FIG. 1, the hard pack battery 10 forms an outer portion of an external device 12 on which the battery 10 is mounted, and thus has advantages such as convenient installation of the battery to the external device (12) upon use. However, this battery type has also problems in that it is relatively expensive and shows less compatibility between various products because the battery case (housing 11) should be designed in compliance with kinds of the corresponding external devices under the condition that the cell body (not shown) is built-in.
Unlike the hard pack type, the inner pack battery 20, as shown in FIG. 2, has advantages such as easy design, low production costs and good compatibility, although it has disadvantages in that it is installed inside the external device and is covered by a case which in turn forms a portion of the external device, thus being relatively cumbersome in installation and use of the battery.
More detailed structure of the inner pack battery 20 is illustrated in FIGS. 3 and 4. As shown in FIGS. 3 and 4, the inner pack battery 20 includes a cell body 21 provided with a positive terminal on the one side thereof and a negative terminal on the other side; a positive temperature coefficient (PTC) element 22 connected to either of two electrode terminals of the cell body 21 and providing primary protection of the battery against overcurrent, overdischarge and overcharge; a protection circuit 24 connected to one electrode terminal (either the positive or anode terminal) on the side of the PTC element 22 via a nickel plate 23 and connected to the other electrode terminal via a nickel plate 27 to provide secondary protection of the battery, and having an external input/output terminal formed on the outside thereof to which the corresponding device (not shown) can be connected; top and bottom cases 25 and 26 enclosing the cell body 21, PTC element 22 and protection circuit 24.
Insulating sheets 28 are disposed between the sides of the cell body 21 and nickel plate 23, and between the protection circuit 24 and nickel plate 27, thus being capable of preventing unnecessary contact between the nickel plates 23 and 27 and adjacent cell body 21 or protection circuit 24.
Further, a double-sided tape 29 is disposed between the cell body 21 and bottom case 26, thus enabling the cell body 21 to be closely fixed on the bottom surface of the bottom case 26. Therefore, the cell body 21 is housed and stably secured within the top and bottom cases 25 and 26.
However, the battery having such a structure has suffers from the following problems.
Firstly, in order to keep pace with the continuing trend towards miniaturization, slimness and weight reduction of external devices, such as miniaturization and weight reduction of the cells, smaller and lighter batteries are in great demand. Achieving this goal requires fabrication of a battery case by way of ultra precision injection molding of thin film which is technically difficult and inevitably causes increased production costs.
Secondly, a large number of processes are required for assembly and mounting of the PTC element, nickel plate, protection circuit, top and bottom cases and assembly processes are complicated, thus resulting in high rejection rate and high production costs.
Thirdly, coupling between top and bottom cases is largely performed by ultrasonic welding, and such ultrasonic welding requires that the battery pack have a size larger than a predetermined thickness. Therefore, this requirement serves as an obstacle to realization of miniaturization, light weight and slimness of the battery, and minute flow of top and bottom cases occurring during ultrasonic welding leads to a high possibility of rejection and the necessity of continued maintenance.
As schemes to solve the above-mentioned problems, a method is proposed involving insert injection molding of various components (cap assembly), which are mounted on the upper part of the cell body, in conjunction with the cell body.
Fabrication of the battery using such insert injection molding does not use separate top and bottom cases and thus it is possible to realize reduction in the size of the battery (in particular, thickness). However, since the cap assembly is molded together with the cell body in a mold, there are required a large number of processes for electrical connection between respective components. Further, there are technical difficulty associated with positioning of respective components in place at predetermined sites in the mold and problems associated with damage and stability of the cell body due to contact with a hot melt resin. In addition, when some components are defective, it is substantially impossible to disassemble the battery structure, thus being incapable of fundamentally solving factors responsible for defective items.